<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div
      class="outer"
      style="
        position: relative;
        height: 300px;
        box-sizing: border-box;
        padding: 10px;
        width: 100px;
        background-color: skyblue;
      "
    >
      <div class="inner" style="position: absolute; height: calc(100%);width: 100%;background-color: orange;top: 0;left: 0;"></div>

    </div>
  </body>
</html>
